Freight and passenger railroad car wheel sets can develop sustained lateral oscillations, commonly referred to as high-speed lateral instability or “hunting”, while operating on railroad track at elevated speeds. The consequences of wheel set lateral instability include:
1. Excessive suspension wear.
2. Damage to lading carried by railroad vehicles, particularly finished automobiles, electronic products or any items that are sensitive to sustained vibrations.
3. Increased derailment risk.
4. Increased fuel consumption of trains with hunting cars.
5. Reduced train operating speeds.
Lateral instability is a natural consequence of the typical railroad car wheel set design (FIG. 1a) that consists of a pair of conical shaped wheels 4,5 mounted rigidly to a solid axle 6. This design is inherently unstable as the wheel set rolls on the rails as shown in FIGS. 1a and 1b. A slight lateral displacement of the wheel set 4, 5, 6 toward the left rail 2 causes the effective rolling radius of the two wheels of the wheel set to change, with the effective rolling radius of the left wheel 4, rleft, increasing and that of the right wheel 5, rright, decreasing. Because the wheels 4,5 are connected via a rigid axle 6, they cannot rotate independently of one another. The difference in their rolling radii (rleft>rright) caused by the lateral shift creates longitudinal and lateral creep forces Fcreep at the wheel/rail contact area 100, 101 that act to restore the wheel set back to its equilibrium position on the rails.
However, due to insufficient damping forces in this simple mechanical system the wheel set will tend to oscillate laterally around its equilibrium position, as shown in FIG. 1a. The magnitude and frequency of this lateral oscillation depends on several factors, including the amount of taper (T1, T2) of the wheel tread cross section, the friction between the wheels 4, 5 and rails 2, 3 the lateral alignment of the railroad track, the design and condition of the railroad car's suspension and, most importantly, the weight and speed of the railroad car, which is shown traveling into the page for FIGS. 1a, 1b. Lateral instability tends to increase as railroad car weight decreases and speed increases.
Railroad cars have suspensions commonly referred to as “trucks” or “bogies”. Several different types of trucks are currently used in railroad cars, but most consist of two or more rigid axle wheel sets contained within a framework that rotates horizontally under the railroad car body to negotiate curves. FIG. 2 shows top views of a typical railroad car truck 7 with laterally unstable wheel sets at five locations L1→L5 along the track. The lateral oscillations of the wheel sets 4,5,6 are shown, and their trajectories 20, 21 are represented as the dashed lines passing through each wheel. L1 shows truck 7 veering left. L2 shows truck 7 veering right. L3 shows truck 7 veering about straight. L4 shows truck 7 starting to veer left again. L5 shows truck 7 returning past straight again before veering right again.
Attempts have been made to minimize wheel set lateral instability in railroad cars by several methods:
1. The use of cylindrical wheel shapes or wheels with very little tread taper.
2. Increasing the yaw resistance of railroad car suspensions to prevent lateral wheel set oscillations.
3. Adding yaw dampers to railroad car suspensions to damp out the lateral wheel set oscillations.
Unfortunately these methods also tend to degrade the ability of railroad car suspensions to negotiate curves, and they increase the cost and maintenance of railroad car suspensions. Thus, the vast majority of freight railroad cars in service in North America are not equipped with any special equipment to control wheel set lateral instability. As a consequence high-speed instability is remedied by simply replacing wheel sets and truck components when lateral instability is detected.
Truck tracking errors occur when one or more wheel sets in a truck run with a lateral offset toward one rail or the other. The causes of this behavior include:
1. The two wheels of a wheel set have worn to different diameters.
2. Different side/side wheel set center distances (d1, d2) due to defects in the truck frame (FIG. 2, L1)
3. Truck frames 7, locked in misalignment with the railroad car and track due to rotational binding or friction at their pivot point 22 (FIG. 2, L1).
Three truck-tracking situations are illustrated in FIGS. 3a, 3b, 3c. FIG. 3a shows the top view of a truck 7a in proper alignment with the track. Both wheel sets 30, 31 are in rolling alignment with the track and are centered between the rails. FIG. 3b shows a truck 7b that is not tracking properly. The truck center member 22 is locked in a rotated position such that neither wheel set 32, 33 in the truck 7b is aligned with the track. The misalignment causes both wheel sets 32, 33 to track toward the left rail. In FIG. 3c the truck tracking error is characterized by the leading wheel set 35 tracking toward the right rail, and the trailing wheel set 34 tracking toward the left rail.
The current invention utilizes the same array of inductive proximity sensors as the lateral instability detector to detect wheel sets that are tracking toward one rail or the other. The invention also employs an algorithm that evaluates the wheel set trajectory to determine if a wheel set is tracking consistently toward one rail or the other.
Several methods have been previously developed to detect and quantify the lateral instability of railroad cars. Prior art involved placing acceleration or force sensors on individual railroad cars and monitoring these sensors in a series of track tests under controlled conditions. These “on-board” methods of detecting and quantifying lateral instability are not practical for the large number of railroad cars in operation on the freight railroads.
Another lateral instability detection device has been developed for commercial applications by Salient Systems, Inc. This device employs strain gauge force sensors applied to lengths of rail that sense the lateral forces applied by railroad car wheel sets. Proprietary computer algorithms are applied to the wheel set lateral force data to detect lateral force patterns associated with lateral instability.
The lateral force measurement method of detecting lateral instability suffers from the following problems:
1. Lateral force measuring sensors must be applied to the rails and calibrated periodically.
2. The lateral force sensors cannot be removed and reapplied to the rails for track maintenance.
3. Certain track maintenance activities destroy the lateral force sensors.
4. The lateral force sensors are susceptible to voltage surges that propagate along the rails.
5. Lighter railroad cars may generate lateral wheel forces that are below the sensitivity threshold of the sensors and will not be detected even though the railroad car wheel sets are laterally unstable.
The advantages of the lateral displacement measurement method of detecting lateral instability of the present invention compared to the lateral force method include:
1. The lateral displacement sensors of this invention are easily removed from the rails and do not require periodic calibration.
2. The inductive proximity sensors are well isolated from the rails and are less susceptible to damage from voltage surges in the rails.
3. The lateral displacement sensor detection capability is not affected by the magnitude of the lateral wheel force, and very light railroad cars (those more inclined to hunt) are detected as reliably as heavier railroad cars.
The shape of the sinusoidal trajectory of a laterally unstable wheel set is more uniform and easier to characterize compared to the wheel set lateral force time series.
Prior art for detecting truck tracking errors consists of a commercial product offered by Wayside Inspection Devices Inc. (http://www.wid.ca) called the T/BOGI™ system (U.S. Pat. No. 5,368,260). This device consists of a laser/camera range finder system that scans the side of passing railroad car wheel sets to measure their angular orientation and tracking disposition relative to the track.
The disadvantage of this prior art is the complexity and cost of the laser/camera range finder system and the need for periodic cleaning and maintenance. In addition, the T/BOGI™ system obtains one instantaneous measurement of the wheel set tracking position at a single point on the track.
The current invention evaluates the tracking position of the wheel set at several points along the track. Furthermore, the present invention detects light railroad cars, which are most prone to hunt. The present invention is easier to maintain and more resistive to damage caused by voltage surges in the rails.